Non-aqueous electrolyte secondary batteries such as lithium secondary batteries are extensively used as power sources for small portable terminals, mobile communication apparatus, and the like because these batteries have a high energy density and a high voltage can be taken out thereof.
At present, an Li—Co composite oxide having a layer structure belonging to the space group R3-m (hereinafter referred to also as LiCoO2) is mainly used as a positive active material in lithium secondary batteries for small public-use applications. The reasons for this are that the LiCoO2 shows a flat discharge potential profile at around 3.9 V (vs. Li/Li+) and that since the delay grade of lithium ion diffusion in the solid LiCoO2 phase even in the final stage of discharge is small, the discharge polarization accompanying on the delay can be diminished and high energy can be taken out over a prolonged time period. In addition, even when lithium ions are extracted to about 4.3 V (vs. Li/Li+) in the charge direction, the crystal structure retains a rhombohedral crystal and, hence, the composite oxide is excellent also in charge/discharge cycle performance. As described above, the LiCoO2 is a positive active material which has a high energy density and satisfactory charge/discharge cycle performance.
With respect to expressions of space groups, the expression “R3m” should originally bear a bar (lateral line) affixed over the numeral “3”. In this description, however, the expression “R3-m” is used for convenience so as to show the same meaning.
Positive active materials likewise having an operating potential at around 4 V include lithium nickelate (LiNiO2). However, the LiNiO2 has problems that it has a lower discharge potential than the LiCoO2 although it has a high theoretical discharge capacity, and that the active material has poor thermal stability in charged state. There are hence many unsolved problems in putting this active material into practical use.
Lithium manganate (LiMn2O4) also can be made to have an operating potential around 4 V and to show a flat discharge potential profile by partly displacing the manganese sites by lithium. However, there has been a problem that this active material has a low theoretical discharge capacity and a problem that a manganese species dissolves away from the active material in a high-temperature environment and this causes a decrease in battery performance. It is explained that the problem of this manganese species dissolution is attributable to the Jahn-Teller strain of trivalent manganese.
Many investigations are being made also on materials represented by the chemical composition formula LiCoxNi1−xO2. This kind of active material is thought to be based on the idea that the merits of both of two active materials, i.e., LiCoO2 and LiNiO2, are imparted by utilizing the fact that LiCoO2—LiNiO2 forms a solid solution throughout the whole compositional range. However, due to the introduction of nickel, this active material also has poorer thermal stability during charge than the LiCoO2 described above. In this point, this active material is not superior in properties to the LiCoO2.
Since the LiCoO2 shows most satisfactory performances among the currently known active materials of the 4-V class, it has come to be used almost exclusively in the market for small public-use appliances. However, the recent trend toward higher performances in small communication apparatus is remarkable and there is a strong desire for a further improvement in battery performance.
For the purpose of further improving the properties of the LiCoO2 described above, a technique in which cobalt sites in the crystal structure of the LiCoO2 are displaced by the element of Zn, Cu, Si, Mg, P, Be, Ca, Sr, Ba, or Ra is reported in Japanese Patent No. 3,162,437 and a technique in which the sites are displaced by aluminum element is reported in JP-A-11-7958 and JP-A-11-73958. However, there has been a problem that since these displacing elements undergo no electrode reaction at around 4 V, the presence of these displacing elements in the active material reduces the discharge capacity. Especially when aluminum element is contained in the active material, there has been a problem that the presence thereof lowers the bulk density of the active material and hence reduces the energy density of the battery.
There has further been a problem that when the LiCoO2 is charged at a high temperature, lithium ions tend to be excessively extracted. When excessive extraction of lithium ions occurs during charge, the negative-electrode side functions to incorporate the excess lithium thereinto. That part of the lithium which remains undoped during this charge is thought to deposit as lithium metal on the negative electrode. The lithium metal which has thus deposited hardly redissolves to become utilizable as an active material. Because of this, not only the battery capacity decreases, but also there has been a possibility that the lithium deposited might penetrate the separator to cause internal short-circuiting.
On the other hand, in large batteries intended to be used in electric motorcars or power storage, the active materials to be used in the electrodes are required to have high thermal stability, for example, because the electrodes have a large size and the batteries are susceptible to heat buildup. Consequently, use of lithium-nickel oxides or lithium-cobalt oxides as a positive active material for such large batteries is avoided, and a lithium-manganese oxide having a spinel structure tends to be employed as the positive active material for the batteries because of its high thermal stability.
However, the lithium-manganese oxide having a spinel structure has had problems that the energy density thereof per unit weight is as low as about 70% of that of the lithium-cobalt oxides, and that with respect to storage performance and charge/discharge cycle performance, the rate of active-material deterioration represented by the deactivation accompanying Mn2+ dissolution is high, resulting in a short battery life.
From these standpoints, a technique of using a positive electrode comprising a mixture of a lithium-manganese oxide having a spinel structure (Li1+xMn2O4 or Li1+xMn2−zMzO4), LiCoO2, LiCo1−xNixO2, and polyaniline for the purpose of improving energy density is disclosed in JP-A-2001-319647. A feature of this technique resides in that the amount of lithium which is deactivated by, e.g., film formation on the negative electrode during first charge can be compensated for by mixing the lithium-manganese oxide with LiCo1−xNixO2, which has a high charge capacity, and with polyaniline and, hence, an increase in battery capacity can be attained. However, since the LiCo1−xNixO2 has insufficient thermal stability, use of this positive electrode in large batteries has been problematic even when the proportion of the oxide is small. There has further been a problem that since polyaniline has a charge/discharge reaction region around 3 V, the polyaniline hardly contributes to charge capacity in that combination, which is intended to be mainly used in a 4-V region.
Furthermore, JP-A-2002-100358 discloses a technique in which a mixture of a lithium-nickel-manganese-M composite oxide represented by LixNiyMn1−y−zMzO2 (wherein x is 0.9≦x≦1.2, y is 0.40≦y≦0.60, z is 0≦z≦0.2, and M is one member selected from Fe, Co, Cr, and Al atoms) and a lithium-manganese spinel composite oxide having an Fd3m spinel structure and represented by LipMn2O4 (wherein p is 1≦p≦1.3) is used. However, there has been a problem that since the LixNiyMn1−y−zMzO2 is an active material having poorer high-rate discharge performance than the lithium-manganese oxide having a spinel structure as pointed out in that patent document, a lithium secondary battery having excellent high-rate discharge performance cannot be obtained.
An object of the invention, which has been achieved in view of the problems described above, is to provide a positive active material which can give a battery having a high energy density and excellent high-rate discharge performance and inhibited from decreasing in battery performance even in the case of high-temperature charge, and to provide a non-aqueous electrolyte battery employing the positive active material.
Another object of the invention is to provide a non-aqueous electrolyte battery which retains the high thermal stability characteristic of lithium-manganese oxides having a spinel structure, has a high energy density and excellent high-rate discharge performance, is inhibited from suffering self-discharge, and has excellent storage performance.